This invention relates generally to data entry using a keypad and in particular to the entry of text characters using a numeric keypad.
Portable telephones, such as cellular and satellite telephones are increasingly being used for a variety of applications, such as internet access, electronic mail, text messaging, data storage and retrieval, electronic commerce etc. These applications require that textual information be entered by the user. Generally, handheld devices are too small to incorporate a full alpha-numeric keypad and instead have a numeric keypad together with a limited number of additional keys. It is common for the numbers 2-9 of the numeric keypad to be used to enter text. Since these eight keys must represent all 26 letters of the alphabet, each key has to represent several letters. For example, it is common for the numbers 2-6 and the number 8 to each represent 3 letters, while the number 7 and 9 each represent 4 letters.
A commonly used scheme is to press each key a specified number of times to select a letter. For example, the xe2x80x9c2xe2x80x9d key is used to represent the letters xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d. To enter the letter xe2x80x9caxe2x80x9d, the key is pressed once, to enter xe2x80x9cbxe2x80x9d the key is pressed twice and to enter xe2x80x9ccxe2x80x9d the key is pressed 3 times. The letter sequence xe2x80x9cbaxe2x80x9d and the letter xe2x80x9ccxe2x80x9d both require 3 key presses. In order to differentiate between these two cases, an additional key, usually referred to as the  less than enter greater than  key must be pressed after each letter is selected, i.e. xe2x80x9cbaxe2x80x9d is entered as the key sequence xe2x80x9c2xe2x80x9d, xe2x80x9c2xe2x80x9d,  less than enter greater than , xe2x80x9c2xe2x80x9d,  less than enter greater than . The letter xe2x80x9ccxe2x80x9d is entered as xe2x80x9c2xe2x80x9d, xe2x80x9c2xe2x80x9d, xe2x80x9c2xe2x80x9d,  less than enter greater than .
If all letters were equally likely to be entered, an average of three keystrokes would be required for each letter. This is clearly a much less efficient way of entering text than using an alpha-numeric keyboard, which only requires a single keystroke to enter each letter.
In U.S. Pat. No. 6,150,962, an alternative scheme is described. Here, it is recognized that letters are not equally likely to be used. Some letters occur more often than others. In addition, the sequence of letters is not random. The probability of the next letter being an xe2x80x9caxe2x80x9d, for example, depends upon what letters have been entered previously. In U.S. Pat. No. 6,150,962 a look-up table is used to store the next most likely letter in a sequence. The table is indexed by the two prior letters entered. In this approach, the order of letters associated with a numeric key is not fixed. Rather, the first letter is determined by using the two previous letters to index the look-up table and determine the next most likely letter. This information must be passed to the user, so this approach requires that the device have a text display, so that the user can see which letter has been selected. The order of the remaining letters for each key is fixed and does not depend upon the prior letters. The device in U.S. Pat. No. 6,150,962 also uses a  less than rotate greater than  key rather than an  less than enter greater than  key. While this approach does, on average, reduce the number of key strokes required to enter text, it has several disadvantages. Firstly, the use of a  less than rotate greater than  will not be familiar to most users, and secondly, the look-up table provides only the most probable letter. If this is not the correct letter, the next letter presented may be one with a very low probability. This feature reduces the efficiency of the method.
It is accordingly an object of the invention to be able to provide a method for entering text that reduces the number of key presses required, is reliable, and is easy to use.